When connecting or forward-driving through a diode, at first the junction capacitance of the diode must be discharged and subsequently the flow capacity of the diode must be charged. On the other hand, when blocking the diode, this procedure must be reversed. Due to the geometry of their conductor lines, electric circuits always have an inductance, which forms a resonant circuit with the capacitances of the diode. In addition, the electric circuit can contain further inductances, for example in switching regulators, which cause an inductance increase in the electric circuit. The resonant circuit formed by the inductances and capacitances has one or more resonant frequencies, which as a rule are considered to be disturbing or interfering. Particularly disturbing are resonant frequencies of resonant circuits that are formed from an inductive electric circuit with a periodically connected or switched diode.
In order to reduce the effects of resonant frequencies, it is known to connect in series a capacitor and an ohmic resistor parallel to the diode. By the higher total capacitance of the electric circuit, the resonant frequency is shifted towards lower frequencies, where it is considered to be less disturbing. The quality of the resonant circuit is reduced by the additional capacitor and the ohmic resistor.
It is disadvantageous with the described series connection of a capacitor and an ohmic resistor that a detuning of the resonant circuit is possible only to a limited extent. The ohmic resistor must be selected to be so low that the amount of the impedance of the series connection of capacitor and ohmic resistor is in the range of the amount of the impedance of the diode capacity. Thus the value of the ohmic resistor is limited to a few ohms. In addition the capacitance of the capacitor may not be selected to be too high, as the capacitance must be charged and discharged with the periodic switching of the voltage at the diode, which leads to high short-circuit starting currents with a high capacitance.